Cryopumps currently available, whether cooled by open or closed cryogenic cyles, generally follow the same design concept. A low temperature array, usually operating in the range of 4 to 25K, is the primary pumping surface. This surface is surrounded by a higher temperature radiation shield, usually operated in the temperature range of 70 to 130K, which provides radiation shielding to the lower temperature array. The radiation shield generally comprises a housing which is closed except at a frontal array positioned between the primary pumping surface and the chamber to be evacuated. This higher temperature, first stage frontal array serves as a pumping site for higher boiling point gases such as water vapor.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array and into the volume within the radiation shield and condense on the lower temperature array. A surface coated with an adsorbent such as charcoal or a molecular sieve operating at or below the temperature of the colder array may also be provided in this volume to remove the very low boiling point gases such as hydrogen. With the gases thus condensed and/or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.
In systems cooled by closed cycle coolers, the cooler is typically a two stage refrigerator having a cold finger which extends through the rear of the radiation shield. The cold end of the second, coldest stage fo the cryocooler is at the tip of the cold finger. The primary pumping surface, or cryopanel, is connected to a heat sink at the coldest end of the second stage of the cold finger. This cryopanel may be a simple metal plate or an array of metal baffles arranged around and connected to the second stage heat sink. This second stage cryopanel also supports the low temperature adsorbent. The radiation shield and frontal array are connected to a heat sink, or heat station, at the coldest end of the first stage of the refrigerator.
After several days or weeks of use, the gases which have condensed onto the cryopanels, and in particular the gases which are adsorbed, begin to saturate the system. A regeneration procedure must then be followed to warm the cryopump and thus release the gases and remove the gases from the system.
For many operations the extremely low pressures provided by the cryopump may be lower than desired. For example, for best results in sputtering processes pressures of inert gases in the range of 1.times.10.sup.-4 torr to 5.times.10.sup.-2 torr may be required. Conventional cryopumps operate most efficiently at pressures below 1.times.10.sup.-5 torr.
In conventional systems utilizing cryopumps to create proper conditions for sputtering, inert gases such as argon are injected into the work space during the sputtering operation to raise the work space pressure and provide an inert gas environment. The specific pressure desired is obtained by a balance of argon introduced into the work chamber and argon condensed by the cryopump. It is evident that cryopump regeneration need occur more often if large amounts of inert gas are injected into the environment during operations.
In some systems a throttle valve has been positioned between the cryopump and the work space. The throttle valve serves to create a pressure differential between the work space and the cryopump by restricting the gas flow between the two. By varying the restriction of the throttle valve, the pressure in the work chamber can be varied while minimizing the flow of inert gas into the chamber and ultimately to the cryopump.
Throttle valves add to the complexity of the system and have not been completely successful. Throttle valves held at ambient temperatures may restrict flow of water vapor and the like to the cooled surfaces which capture the water vapor. Such restriction of flow of undesired gases as well as flow of inert gases may result in contamination of the work space. To avoid that problem, frontal valves are more usually cooled to condense and retain the water vapor upstream of the flow restriction. As such, the throttle valves may perform as second stage arrays. A disadvantage of cooled throttle valves is that the condensed water vapor can interfere with the mechanism of the throttle valve and thus prevent or limit its operation after cooldown of the system. Also, the valve adds undesired complexity to the system.
As an alternative to a variable throttle valve, a restriction in the form of a cooled orifice plate has been described in my prior U.S. Pat. No. 4,449,373. In that system, a plate cooled by the first stage of a cryogenic refrigerator has a plurality of circular orifices which restrict flow of gas from the work chamber into the cryopump. That approach has the advantage of structural simplicity with no moving parts.